Organofunctional silanes are well known in plastic-forming technology as excellent coupling agents for bonding organic resins to reinforcement materials. Ordinarily, reinforcement materials are coated with and bonded to coupling agents and then intimately blended with the impregnating resins which in turn bond to functional groups on the coupling agents. Selection of the proper organofunctional silane is usually determined by choosing a coupling agent with a functional group that is capable of undergoing a known polymerization condensation with one or more of the functional groups of the impregnating resin. Proper matching of silane organic functional groups with those of the polymer resin is essential to the formation of a strong, thermally-resistant chemical linkage of the two materials. Polyolefins are especially difficult to reinforce with hydroxyl-containing reinforcements using most conventional silane coupling agents. This is evidenced by a lower than optimum property profile, especially resistance to impact and lowered heat deflection temperatures.
Various functional silanes, e.g., glycidoxypropyltrimethoxysilane and methacryloxypropyltrimethoxysilane have been used for many years to improve the adhesion of organic resins to inorganic reinforcements, like glass fibers, silica, wollastonite, clays, mica, asbestos and other hydroxyl-containing materials. The silanes hydrolyze and attach themselves to the surface hydroxyl groups leaving a polymer-compatible terminal function. Usually from about 0.05 percent to about 5 percent by weight of the reactive silane coupling agent based on the reinforcement additive is used.
Such a technique is not free from problems when polyolefins, such as polyethylene and polypropylene are used. Polyolefins blended with conventional silane-treated hydroxyl-containing fillers are difficult to mold because they have reduced flow, requiring higher molding pressures, and the resulting impact strengths and heat distortion properties are not optimum.
There still exists a need to provide improved properties of the overall composition. Specific types of property improvements are viscosity reduction, increased physical strength and thermal resistance. The prior art discloses several types of coupling agents tailored for use with polyolefin systems. In an early development, Bixler et al., U.S. Pat. No. 3,471,439, proposed a three-part system, which contained:
(1) an organic compound having a chemical affinity for the surface, which can be, but not necessarily is, one of the conventional organofunctional silanes mentioned above;
(2) an organic compound with at least two (2) polymerizable ethylenic unsaturations, such as 1,3-butylene glycol dimethacrylate; and
(3) a free-radical generator such as an organic peroxide catalyst.
This three-part composition must be added when the filler is mixed with the matrix polymer. If the filler is to be treated prior to addition into the polymer matrix, however, two free-radical catalysts must be used. The first is a low-temperature free-radical generator required partially to react and bond the unsaturated material to the surface. The second is a high-temperature free-radical generator which will remain on the surface to react residual unsaturated material with the polymer matrix, at the time the polymer is processed. The high temperature free-radical generator must be tailored to the polymer material, in order to match the processing temperature associated with the polymer. The system is undesirably complex and commercially difficult to carry out.
Godlewski et al., U.S. Pat. No. 4,481,322, disclose a four-component system that can be added at the most convenient and economical point in the overall formulation procedure, i.e., either pretreating the filler or blending everything in the extruder hopper or injection molding machine hopper. However, in order to provide the improved physical and processing properties of Godlewski et al., the following four components are disclosed to be essential:
(1) a polymerizable unsaturated organic compound containing at least two polymerizable unsaturated groups, e.g., trimethylol propane triacrylate;
(2) a vinyl polymerizable unsaturated, hydrolyzable silane, e.g., gamma-methacryloxypropyl trimethoxysilane;
(3) a surfactant, such as a polysiloxane; and
(4) a free-radical generator such as an organic peroxide.
The inclusion of a surfactant is an integral and necessary part of the Godlewski et al. compositions. A significant commercial drawback of such compositions is that the peroxide (4) and the polymerizable unsaturated organic compound (1) must be kept separate prior to actual blending into polymer matrix or treatment onto the surface of fillers. Therefore, its use requires that two or more of the components be added at the time of incorporation.
Gaylord, U.S. Pat. No. 4,317,765, discloses a two-part coupling agent that does not include a silane but is reported to provide polyolefin compositions having improved physical properties. The Gaylord composition is comprised of an ethylenically unsaturated carboxylic acid or anhydride, illustratively maleic anhydride, and a free-radical generator, such as an organic peroxide.
According to the Gaylord disclosure, the coupling agent may be incorporated by a one-step method directly into the polymer composition, or by a two-step method whereby the filler particles are encapsulated with a polymer material via the coupling agent. The encapsulating polymer may or may not be the same as the matrix polymer in the final composition. Preferably, the coating and matrix polymer are of the same chemical species, however, with the matrix polymer having a lower melt index and higher molecular weight than the coating material. In either the one-step or two-step process, the ethylenically unsaturated material, free-radical generator and polymer must be added simultaneously to produce the beneficial results, making the mixing requirements and order of addition critical to overall performance and introducing significant commercial processing drawbacks. A five-fold decrease in Izod Impact Strength was observed when orders of addition were varied.
Marzola et al., U.S. Pat. No. 4,429,064, disclose mica-reinforced polyolefin compositions comprising a maleamic silane modifier. The mica is pretreated with from 0.01 to 7 percent by weight of a modifier, typically of formula ##STR1## wherein Z is a(n) aliphatic, aromatic, cycloaliphatic or heterocyclic divalent radical, R is C.sub.1-6 alkoxy, or halogen, and n is 1-3. The modified mica is blended with polypropylene and the physical properties, heat distortion temperature and melt flow are found to be improved somewhat over those obtained with unmodified mica.
The present invention overcomes the problems of the prior art by providing a stable, two-component reinforcing additive composition which can be added to the filler at a convenient point in a single step. It can also be used as pretreatment onto filler particles without a polymer material present to encapsulate the particle and eliminate the need for a surfactant.
Unlike both Bixler et al. and Godlewski et al., there is also no need to include a component having at least two vinyl functions as an essential component. Unlike Godlewski et al. it is unnecessary to include a surfactant as an essential component, and the two component system discovered herein provides better heat distortion temperature, impact strength and flow properties in polypropylene than does the Godlewski et al. compositions, which represent the current state-of-the-art. In contrast to Gaylord, the present additives do not require critical mixing steps to achieve high, reproducible physical properties. In comparison, with the single component coupling agent of Marzola et al. it has been discovered that inclusion of a free radical generator in a maleamic silane modifier provides a vast and entirely unexpected improvement in ultimate properties and processability of difficult to reinforce polyolefin compositions.